Apparatus for magnetic stirring of continuous castings

ABSTRACT

The molten interior of a continuously cast metal slab is magnetically stirred by mounting inside one or more of the supporting and guiding rolls for the case ingot emerging from the casting mold an electrical inductor connected to a multi-phase current source so that a mobile magnetic field is generated.

United States Patent [1 1 Alberny et al.

[ 51 May 13, 1975 APPARATUS FOR MAGNETIC STIRRING OF CONTINUOUS CASTINGS Inventors: Robert Alberny, Metz; Jean Delassus, Montmorency, both of France Assignees: Institut de Recherches de la Siderurgie Francaise (IRSID), St. Germain-en-Laye; Compagnie Electro-Mecanique (CEM), Paris, both of France Filed: June 4, 1973 Aopl. No.: 366,592

Foreign Application Priority Data June 8, 1972 France 72.20546 June 8, 1972 France 1 72.20545 May 29, 1973 France 73.19399 May 29, 1973 France 73.19400 US. Cl. 164/250; 164/282; 164/49 [51] Int. Cl B22d 11/12 [58] Field of Search 164/49, 82, 146, 250, 282, 164/251 [56] References Cited UNITED STATES PATENTS 3,656,537 4/1972 Von Starck 164/82 X 3,693,697 9/1972 Tzavaras 3,746,074 7/1973 Baumann 164/49 X Primary ExaminerR. Spencer Annear Attorney, Agent, or Firm-Kurt Kelman [57] ABSTRACT The molten interior of a continuously cast metal slab is magnetically stirred by mounting inside one or more of the supporting and guiding rolls for the case ingot emerging from the casting mold an electrical inductor connected-t0 -a multi-phase current source so that a mobile magnetic -field is generated.

8 Claims, 12 Drawing Figures PATENTED MAY I 31975 saw 10? a PATENTED RAY I 31975 SHEEI Q 0F 4 FIG. IO

FIG. I!

APPARATUS FOR MAGNETIC STIRRING OF CONTINUOUS CASTINGS The present invention relates to improvements in a casting apparatus for continuously casting a metal strand or ingot of an elongated transverse cross section,

such as a metal slab which has opposite largefaces,

wherein magnetic stirring of the interior molten portion of the ingot may be effected without reducing the guidmoved away therefrom through a cooling zone, the cast ing continuous casting apparatus includingmeans for the magnetic stirring of a billet. In this apparatus,the'-' molten metal inside the cast billet is rotated by means of a rotating magnetic filed generated by a stator structure surround the billet.

US. Pat. No. 3,693,697 proposes the magnetic stirring of a continuously cast product by means of a socalled travelling magnetic field which induces a fluid flow parallel to the direction of withdrawal of the product from the casting mold, the magnetic field being generated by a plurality of electric coils surrounding thecast product and suitably connected to a source of. multi-phase current.

Previous attempts to apply known methods of magnetic stirring to elongated cast products, such as slabs, have encountered some difficulties resulting from the fact that the outer solid layer or skin of the product emerging from the casting mold is not capable of resisting deformation due to the ferrostatic pressure exerted upon it from the molten interior of the product. Therefore, casting apparatus for the continuous casting of such products generally include a cooling apron with supporting and guiding rolls so closely spaced that they contact the faces of the cast slab below the mold at such closely spaced points that the rolls define. asupport for the solidified skin of the product which has substantially the same geometric configuration as the inside of the mold. Thus, the rolls fully support the skin against deformation, at the same time guiding the cast slab through a treatment or cooling zone in a curvilinear path to bring the slab from a substantially vertical to a substantially horizontal position wherein the slab has been solidified and where it is cut into billets.

It is evident from the above considerations that it is hazardous to remove any of the supporting and guiding rolls in that portion of the treatment zone wherein substantial portion of the interior of the cast product is still liquid and replace them by inductor structures for magnetically stirring this liquid portion in the interior of the product. It is in this zone, however, where the magnetic stirring is of greatest importance.

It is the primary object of this invention to provide a casting apparatus for continuously casting'a metal ingot of an elongated transverse cross section,such as a slab,

ing support for the solidified skin of the ingot.

It is another object of the invention to provide an apparatus of this type without necessitating substantial modifications in the structure of existing and generally conventional continuous casting apparatus.

It is a further object of the present invention to provide means for the magnetic stirring of continuously cast ingots, which permit various types of magnetic flow patterns desirable for the most effective stirring of the molten interior of such castings.

With these and other objects in view, the improvements provided by this invention includes at least one of the supporting and guiding elements in the treatment zone wherein the interior of the cast ingot contains molten metal defining an interior cavity, an electrical inductor structure arranged within the cavity, and a source of multi-phase current. The inductor structure is connected to the current source for generating a mobile magnetic field extending through the supporting an guiding element wherein the inductor structure is arrangedinto the molten interior of the cast ingot for magnetically stirring the molten metal in the interior while the cast; ingot is supported and guided through the treatment zone.

In this manner, the full support of the cast ingot is not disturbed by the magnetic stirring means which is simply incorporated into the support so that the cast product may be effectively supported at any level of the cooling apron while the interior thereof is simulta- '40} generally conventional but incorporating the improvement of the present invention;

neously magnetically stirred at the same level.

The above and other objects, advantages and features of the invention will be more fully understood by reference to the following detailed description of certain now preferred embodiments thereof, taken in conjunction with the accompanying drawing wherein FIG. 1 is a vertical side elevational section of the upper treatment zone of a continuous casting apparatus FIGS. 2 and 3 show axial sections of two embodiments of magnetic stirring means according to this invention;

FIG. 4 is a partial transverse section of one embodiment of the magnetic stirring means of FIG. 2;

FIG. 5 is a section along line V-V of FIG. 4;

FIG. 6 is a transverse section of another embodiment of the magnetic stirring means of FIG. 2;

FIG. 7 is a section along line VIIVII of FIG. 6;

FIGS. 8 and 9 respectively illustrate schematically the distribution of the magnetic fields produced by the embodiments of FIGS. 4 and 6;

FIGS. 10 and 11 illustrate two modifications of magnetic stirring obtainable by the apparatus of FIG. 4; and

FIG. 12 illustrates a mode of magnetic stirring obtainable by the apparatus of FIG. 6.

Referring to the drawing, FIG. 1 shows a continuous casting machine in which an open-ended or flowthrough chilled mold 1 casts a continuous strand of metal which emerges from the bottom exit end of the mold, the cross-section of the strand having an elon- (not shown) engaging the slab further downwards. The rolls 2 are generally idler rolls supported on brackets 3 to forman apron having a curvilinear longitudinal axis co-extensive with the axis of mold l to guide the slab from a substantially vertical position to a substantially horizontal position where the slab is cut into lengths or billets. However, the path defined by the rolls might also be a straight vertical line. During the passage of the slab between the rolls, the slab is cooled by water sprays from nozzles 4. All of this structure may be entirely conventional.

At least one of the supporting and guiding rolls 2 consists of a tubular non-magnetic body 5 machined for example, from a stainless non-magnetic steel tube. Tubular body 5 defines an inner cavity 6 and is maintained in contact with a large face of the slab. The contact occurs substantially without relative sliding since the tu-" bular body 5 of the idler roll freely rotates around its axis during the downward displacement of the slab so that the peripheral speed of the roll is substantially identical with the speed of movement of the slab. An.

inductor structure 7 comrising a plurality of windings associated with magnetic core is coaxially arranged inside the inner cavity 6 of the tubular body 5. The inductor windings are suitably connectedto a source of multiphase alternating current comprising, for example, a multiphase alternator 8 driven by a direct current motor 9 adapted to rotate at adjustable speed. In the drawing, the alternator consists of a three-phase alternator. The magnetic field generated by the inductor structure develops through the tubular body 5 and gives rise inside the metal strand being cast to eddy currents and, therefore, to magnetic forces according to a well-known phenomenon. These forces induce a flow of molten metal inside the strand and, therefore, create the so-called magnetic stirring effect which is well known in the art. Simultaneously, the tubular body 5 acts as supporting and guiding means for the surface of the slab along a generatrix of body 5 contacting the slab surface or skin.

A plurality of rolls of the type described, e.g. comprising a tubular body and an internal inductor structure, may be arranged along the upper portion of the path of the slab, wherein it has not yet been cooled or solidified throughout its cross section. FIG. 1 depicts two pairs of such rolls adjacent each other and respectively arranged on both sides of the slab along its two large faces. Such rolls may be arranged at any level below the mold or at a plurality of different levels depending on the stirring action which may prove desirable.

FIG. 2 shows a first embodiment of the apparatus described with reference to FIG. 1, according to which the two ends of the tubular body 5 are respectively affixed to non-magnetic metal spindles l0, 11 so that body 5 and its spindles are adapted to rotate as an integral structure. The spindles are joumaled in bearings 12 attached to the bracket 3. Each spindle has a cylindrical recess 13 in which one end of the inductor structure 7 is frictionally engaged so as to form an integral rotary structure with tubular body 5 and spindles 10, l 1. Details pertaining to the inductor structure 7 will be described further below. One of the spindles, for example spindle 10, comprises an outwardly extending extrernity 14 having three grooves in which are respectively arranged three suitably insulated collecting rings 15, 16, 17. An axial bore 18 is machined in spindle l0,

4 and communicates at one end with the inner cavity 6, further communicating with the collecting rings 15, l6, 17 through radial bores. The collecting rings are adapted torespectively cooperate with carbon blocks 19, 20, 21 which are forced in a conventional manner against the collecting rings by means of springs (not shown) in order to ensure permanent-contact of the carbon blocks with the collecting ring s. Each carbon block is connected to a respective phase of a threephase alternating current source by means of wire 22.

The collecting rings are connected to the windings of inductor structure 7 by means of wires 23. The collecting rings and the carbon blocks are arranged inside a water-tight casing 24.

FIG. 3 shows a second embodiment of the apparatus described with reference to FIG. 1 according to which the two ends of tubular body 5 are supported on the outer rings of rollerbearings 25, 26 the inner rings of which are supported on shaft 27 affixed to brackets 28, 29 and held against rotation by means of key 30. The shaft 27 is machined in its middle portion in order to receive a tubularly shaped inductor structure 7". The shaft 27 carries re'rnovable abutment member 31 allowing for the mounting and dismounting of the inductor structure 7. The inductor structure is connected to a source of three-phase alternating current bymeans of wires 32 arranged inside a bore machined in the shaft 27 and emerging at the outside. The shaft 27 and the inductor structure 7 form a stationary construction around which the tubular body 5 is adapted to rotate.

The embodiment of FIG. 2 preferred because of its simplicity of construction. i

It must be understood that the tubular bo'cly5 is likely to rotate at a very low angular speed since the speed of the cast product is rather low, for example of themagnitude of 1 m/s. The correct functionning of the collecting device comprising the carbon blocks and collecting rings is, therefore, assured over long periods of time.

FIGS. 4 and 5 show a first embodiment of the construction of the inductor structure 7. According to this embodiment, the inductor structure 7 comprises a magnetic circuit associated with windings adapted to generate a travelling magnetic field.

The magnetic circuit of the inductor structure comprises a magnetic core 33 of cylindrical shape engaged, as previously stated, at both ends in the spindles 10 and 11. The core is machined at its periphery to define a plurality of annular grooves 34 regularly spaced along core 33, FIG. 5 showing a core having twelve grooves.

The magnetic core may be of massive construction but I is advantageously laminated in the following fashion: The core comprises a massive center portion with longitudinal slots 35 in which a plurality of suitably insulated magnetic metal sheets 36 are friction-fitted and embedded, as illustrated in FIG. 4. The annular grooves 34 are preferably machined subsequently to the coneach slot being machined between two longitudinal slots 35 and extending toward the axis of the core over a greater depth than the depth of the annular grooves 34 for a purpose that will be further explained herebelow. FIG. 5 shows a section of the inductor structure along the broken line V-V of FIG. 4 in order to clearly illustrate the construction of the magnetic core.

The windings comprise a plurality of annular conducting coils 38 respectively arranged within each of the annular grooves 34. In the specific case of the described structure, each annular groove is adapted to receive a group of four conducting wires forming a coil connected to a phase of the three-phase alternating current source. The coil inserted in an end groove 34a is connected to a first phase of the current source; the coil inserted in the adjacent groove 34b is connected to a second phase of the current source; the coil inserted in the following groove 340 is connected to the third phase of the current source. The coil inserted in groove 34a is connected to the coil inserted in a fourth groove 34d by means of a wire 39 longitudinally arranged at the bottom of a longitudinal slot 35; the coil inserted in the second groove 34b is connected to the coil inserted in a fifth groove 34e by means of a wire 40 longitudinally extending at the bottom of another longitudinal slot 35; the same arrangement applies to the coil inserted in groove 340 which is connected to the coil inserted in a sixth groove 34f by means of a wire 41 extending longitudinally at the bottom of still another longitudinal slot 35, and soon. It results from these electrical connections that the successive coils of the inductor structure along said structure are connected to the successive phases of the current source. his well known in the art that such a windingarrangement generates a travelling field, the direction of displacement of said field being parallel to the axis of the coil. It must be understood that the winding which has been hereabove described is not limitative and that any known type of winding giving rise to a travelling field and comprising a plurality of annular electric coils following each other and suitable connected to a source of current may be employed within the scope of this embodiment of the present invention. Further details relating to the distribution of magnetic fields will be given in order to illustrate in generic terms the type of travelling magnetic field generated by the inductor structure according to the embodiment described with reference to FIGS. 4 and 5.

FIGS. 6 and 7 show a second embodiment of the inductor structure 7 which comprises a magnetic circuit associated with windings adapted to generate a rotating magnetic field.

The magnetic circuit comprises a magnetic core 42 of cylindrical shape, said core being machined at its periphery to define a plurality of longitudinally extending grooves 43. In the specific case, the magnetic core comprises twelve longitudinal groov'es arranged in six pairs of diametrically opposed grooves. The magnetic core represented on the figures is amassive core but it is also possible to use a laminated construction; in the case of a rotating magnetic field, such a laminated construction shouldinclude a plurality of adjacent insulated magnetic metal sheets, each sheet being arranged in conventional manner in a plane perpendicular tothe longitudinal axis of the magnetic core. g

The winding comprises a lurality' of longitudinally extending wires 44 respectively arranged within each of the longitudinal grooves 43. In the specific case described, each groove is adapted to receive a group of four wires connected to a phase of a three-phase alterning current source. The wires that are arranged in diametrically opposed grooves are connected in such a manner that the wires arranged in one groove form the return wires of the wires arranged in the opposed groove, said wires thus forming together an elongated coil. It is well known in the art that the winding hereabove described is adapted to generate a rotating magnetic. field when the coils are suitably connected to a source of multiphased current, the number of magnetic poles depending on the type of connection selected. In the specific case described, a two-pole inductor structure has been selected. However it should be understoodthat any winding comprising a plurality of longi-. tudinally extending wires inserted within a plurality of longitudinal grooves and adapted to generate a rotating magnetic field might be used within the scope of this embodiment of the present invention. It should be noted as is known in the art, that a suitable concentration of the magnetic field at a distance from the inductor structure has to be realized in order to obtain a magnetic stirring action on the molten part of the ingot, this distance depending on the level at'which the magneticstirring treatment takes place in the continuous casting machine.

FIG. 8 illustrates schematically the distribution of the travelling magnetic field generated by the inductor structure described with reference to FIGS. 4 and 5. This distribution may be represented by the magnetic flux lines at agiven instant t when the coils are suitably connected tothe successive phases of athree-phase alternating current source. In a conventional fashion, the flux lines issuing from the inductor structure form a north pole and the flux lines entering the inductor structure form a south pole. A plurality of successive north and south magnetic poles are thus formed, said poles having at the given instant t a position as represented on the drawing of FIG. 8. The instant t is the instant when the current has a maximum value in the phase connected to the coils respectively arranged in the annular grooves 34b, 34c, etc At this instant, the current value in the coils respectively arranged in the annular grooves 34a, 34c, 34d, 34f etc is half the value of the current in the coils respectively arranged in grooves 34b, 342, etc this being due to the phase angle of between the successive phases of the three-phase current source.

At a following instant t+l, when the current value is at a maximum in the coils respectively arranged in the grooves 34c, 34f, etc a displacement of the successive magnetic poles occurs towards the right side of the figure, and so on at an instant t+2, etc The magnetic field thus created is called in a conventional manner a travelling field. If it is desirable to obtain a displacement of the poles towards the left side of the figure, the coils need only be connected to the current source in order that the current value be at a maximum at the instant t+1 in the coils respectively arranged in the annular grooves 34a, 34d, etc This may be readily ob- .tained by inverting the connections of any pair of phaaxis of the inductor structure but it must be understood that this distribution is identical in all planes containing this axis/Consequently, there is no difference as regards the distributionof the magnetic field whether the inductor structure rotates or does not rotate around its longitudinal axis, the stirring effect inside the product remaining the same in both cases.

FIG. 9 illustrates schematically the distribution of the two-pole rotating magnetic field generated by the inductor structure described with reference to FIGS. 6 and 7. The longitudinal wires 44 arranged inside the longitudinal grooves 43 are connected in a well known manner to a three-phase alterning current source in order to form a north pole and a south pole which have been represented on the drawing at a given instant. It is also well known that the vector representative of the magnetic field rotates in a plane perpendicular to the longitudinal axis of the inductor structure,.the north and south poles rotating in the same direction. The drawing illustrates the distribution of the magnetic field at a given instant in a plane perpendicular to the Longitudinal axis of the inductor structure but it must be understood that this distribution remains identical in all planes perpendicular to this axis along the inductor structure. The direction of rotation of the magnetic field depends on the connectionsfselected between the current source and the wires, and this direction of rotation may be selected at will. The rotation may have the same direction as the rotation of the tubular body or may be opposed to said rotation of the tubular body 5. The inductor structure may or not rotate around its longitudinal axis, the angular velocity of the inductor structure being very small, as previously stated, while the angular velocity of the magnetic field is rather high even for the relatively low frequencies generally employed in the field of magnetic stirring, these fequencies being generally comprised within the range of 10 60 Hz in order to limit inductive heating phenomena.

FIG. 10 illustrates a first mode of stirring which may be obtained by means of the apparatus described with reference to FIGS. 1, 4 and 5.. According to this first mode, the respective travelling fields generated by two inductor structures arranged inside a pair of tubular bodies 5a, 5b are displaced in opposed directions. It is known that the effect of the magnetic fields inside the product, i.e. a slab is such that magnetic forces oriented in the same direction as the direction of displacement of the flux lines develop inside the product, these magnetic forces giving rise to a flow of the molten metal substantially as represented in FIG. 10, the molten metal being displaced substantially parallel to the large faces of the slab in an overall circulatory motion.

Referring now to FIG. 11, there is illustrated a second mode of stirring, which may be obtained by means of the apparatus described with reference to FIGS. 1, 4 and 5 when the two respective travelling fields generated by the two inductor structures respectively arranged inside the tubular bodies 5a, 5b. are displaced in the same direction. In this case, the return flow of the molten metal occurs in the middle portion of the slab cross-section. It should be noticed that the actual flow inside the cast product may be relatively complex since natural convection interfers with the stirring motion resulting from the magnetic fields. The portion of the product which is affected by the magnetic stirring is not strictly limited to the cross-sectional plane, as represented in FIGS. 10 and 11, since the flux lines actually sweep through the product in adjacent portions located above and below said plane. The portion of the product .submitted to magnetic stirring may be extended at will by providing a plurality of successive stirring rolls, as represented in FIG. 1.

A phase reversing switch 45 has been showing FIGS.

10 and 11, which makes it possible to obtain the desirrections on both sides of the molten portion is obtained.

Referring to FIG. 12, a mode of stirring as it may be obtained by means of the apparatus described with reference to FIGS. 1, 6 and 7 has been represented. The

rotating fields respectively generated by four inductor structures respectively arranged inside four tubular bodies 50, 5d, 5e, 5f comprising two pairs of bodies arranged on both sides of a slab are made to rotate in 0p-- posite direction to the rotation of the tubular bodies.-

The relative flow of the molten portion inside the slab L is an ascendingflow, as illustratedin FIG. '12, this ascendin'g flow combining with the natural eonvt-iction gion of treatment. The determination of the various parameters related to the stirring treatment effected on the product does not form a feature of the present invention and may be established on the basis of conventional knowledge in the art of magnetic stirring as applied to the continuous casting technique as well as through direct experimental work. The operator may select in each specific casting machine the level at which the treatment should be applied and the characteristics of the magnetic field at that level.

The apparatus of this invention may be used for the magnetic stirring of continuously cast ingots the crosssection of which presents an elongated shape, for example slabs. It is not limited to cases where the generatrix of the tubular body is a straight line but may be readily adapted to applications where the ingot presents an oblong shape, for example for the treatment of oval-shaped slabs; nor is it limited to such methods of casting where the mold is in a vertical or substantially vertical position but may include such methods where the mold is horizontal or substantially horizontal as in known methods for horizontal continuous casting whenever a stirring treatment might prove desirable in such cases.

We claim:

1. In a casting apparatus for continuously casting a metal ingot of an elongated transverse cross section,

, guiding the cast ingot through a treatment zone wherein the interior of the cast ingot contains molten metal, the improvement including '1. at least one of thesupporting and guiding elements defining an interior cavity,

2. an electrical inductor structure arranged within the cavity, and

3. a source of multi-phase current,

a. the inductor structure being connected to the current source for generating a mobile magnetic field extending through the supporting and guiding element wherein the inductor structure is arranged into the molten interior of the cast ingot for magnetically stirring the molten metal in the interior while the cast ingot is supported and guided through the treatment zone.

2. In the casting apparatus of claim 1, the supporting and guiding element comprises a tubular body of nonmagnetic material and of circular cross section, the interior cavity extending axially within the tubular body and the inductor structure being arranged coaxially within the axial cavity.

3. In the casting apparatus of claim 2, all the supporting and guiding elements being rolls and the tubular body being of the identical exterior shape as adjacent ones of the rolls.

4. In the casting apparatus of claim 2, the inductor structure comprising a magnetic core defining a plurality of annular peripheral grooves axially spaced from one another, and annular coils being arranged coaxially within respective ones of the annular grooves, the coils being so connected to the current source as to generate a magnetic travelling field, the direction of displacement of the magnetic travelling field being parallel to the generatrix of the tubular body of the supporting and guiding element.

5. In the casting apparatus of claim 4, a phasereversing switch arranged between the current source and the inductor structure, the switch being connected to two phases of the inductor structure to permit reversal of the direction of displacement of the travelling magnetic field.

6. In the casting apparatus of claim 2, the inductor structure comprising a magnetic core defining a plurality of longitudinal peripheral grooves angularly spaced from one another, and longitudinally extending wires arranged within respective ones of the longitudinal grooves, the wires being so connected to the current source as to generate a rotating magnetic field, the vector representative of the rotating magnetic field being displaced in a plane perpendicular to the longitudinal axis of the tubular body.

7. In the casting apparatus of claim 2, further comprising a pair of spindles carrying the tubular body and the inductor structure, and respective journals freely rotatably supporting a respective one of the spindles.

8. In the casting apparatus of claim 2, further comprising a stationary shaft, the inductor structure being affixed to the shaft, and roller bearings freely rotatably supporting the tubular body for free rotation about the stationary shaft and inductor structure, 

1. In a casting apparatus for continuously casting a metal ingot of an elongated transverse cross section, which comprises an open-ended casting mold and a plurality of supporting and guiding elements arranged in succession adjacent the mold for engaging respective ones of the ingot faces as the cast ingot emerges from the mold, the succession of elements supporting and guiding the cast ingot through a treatment zone wherein the interior of the cast ingot contains molten metal, the improvement including
 1. at least one of the supporting and guiding elements defining an interior cavity,
 2. an electrical inductor structure arranged within the cavity, and
 3. a source of multi-phase current, a. the inductor structure being connected to the current source for generating a mobile magnetic field extending through the supporting and guiding element wherein the inductor structure is arranged into the molten interior of the cast ingot for magnetically stirring the molten metal in the interior while the cast ingot is supported and guided through the treatment zone.
 2. an electrical inductor structure arranged within the cavity, and
 2. In the casting apparatus of claim 1, the supporting and guiding element comprises a tubular body of non-magnetic material and of circular cross section, the interior cavity extending axially within the tubular body and the inductor structure being arranged coaxially within the axial cavity.
 3. a source of multi-phase current, a. the inductor structure being connected to the current source for generating a mobile magnetic field extending through the supporting and guiding element wherein the inductor structure is arranged into the molten interior of the cast ingot for magnetically stirring the molten metal in the interior while the cast ingot is supported and guided through the treatment zone.
 3. In the casting apparatus of claim 2, all the supporting and guiding elements being rolls and the tubular body being of the identical exterior shape as adjacent ones of the rolls.
 4. In the casting apparatus of claim 2, the inductor structure comprising a magnetic core defining a plurality of annular peripheral grooves axially spaced from one another, and annular coils being arranged coaxially within respective ones of the annular grooves, the coils being so connected to the current source as to generate a magnetic travelling field, the direction of displacement of the magnetic travelling field being parallel to the generatrix of the tubular body of the supporting and guiding element.
 5. In the casting apparatus of claim 4, a phase-reversing switch arranged between the current source and the inductor structure, the switch being connected to two phases of the inductor structure to permit reversal of the direction of displacement of the travelling magnetic field.
 6. In the casting apparatus of claim 2, the inductor structure comprising a magnetic core defining a plurality of longitudinal peripheral grooves angularly spaced from one another, and longitudinally extending wires arranged within respective ones of the longitudinal grooves, the wires being so connected to the current source as to generate a rotating magnetic field, the vector representative of the rotating magnetic field being displaced in a plane perpendicular to the longitudinal axis of the tubular body.
 7. In the casting apparatus of claim 2, further comprising a pair of spindles carrying the tubular body and the inductor structure, and respective journals freely rotatably supporting a respective one of the spindles.
 8. In the casting apparatus of claim 2, further comprising a stationary shaft, the inductor structure being affixed to The shaft, and roller bearings freely rotatably supporting the tubular body for free rotation about the stationary shaft and inductor structure. 